Resin molded articles having a three-dimensional shape or complex shape are molded typically by injection molding. Injection molding can mass-produce molded articles having a desired shape. However, in order to manufacture molded articles that are required to have a high dimensional precision by injection molding, an expensive die having a high dimensional precision is required. Furthermore, since injection-molded articles are readily deformed by shrinkage and/or residual stress after the injection molding, the shape of the die needs to be adjusted precisely depending on the shape of the molded article and properties of the resin material. Since fraction defective is high in injection molding, product cost thereby is often high. Furthermore, injection molding of a molded article having a large thickness is difficult due to shrinkage and/or residual stress.
In order to obtain molded articles having a three-dimensional shape or complex shape, a method for molding a secondarily molded article having a desired shape, the method comprising: extruding and solidifying a resin material; producing a stock shape for machining (also referred to as “stock shape for cutting”) having a shape, such as a plate, round bar, pipe, special shape, or another shape; and subjecting the stock shape for machining to machining, such as cutting, drilling, and shearing, has been known. The method of machining the stock shape for machining has advantages, including that a molded articles can be produced in small quantities at a relatively low cost because an expensive die is not required, that frequent modifications in molded article specifications can be accommodated, that molded articles with high dimensional precision can be obtained, that molded articles having a complex shape or large thickness, which is not suitable for production using injection molding, can be produced, and the like.
However, not all resin materials and/or extrusion molded articles are suitable as stock shapes for machining. A stock shape for machining needs to satisfy high levels of required properties, such as having a large thickness and excellent machinability, having low residual stress, being capable of avoiding excessive heat generation that leads to deformation and/or discoloration due to heat of friction generated during machining, being capable of being machined with high precision, and the like.
In general, most of processing methods used in metallic materials are utilized in machining of polymeric stock shapes as is. Even among extrusion molded products, an extrusion molded product that is thin and has great flexibility, such as a typical film, sheet, or tube, is unsuitable for machining such as cutting. Even among extrusion molded products having shapes, such as plate or round bar, with a large thickness or large diameter, if the extrusion molded product has excessively large residual stress during extrusion molding, the extrusion molded product readily deforms during or after machining, and it is difficult to obtain a secondarily molded article having high dimensional precision. Even among extrusion molded products having reduced residual stress, the extrusion molded product that readily breaks or cracks during machining, such as cutting, drilling, and shearing, is not suitable as stock shape for machining.
In order to obtain, via extrusion molding, a stock shape for machining having properties suitable for machining, selection of resin materials, method of extrusion molding, or the like needs to be devised. Therefore, various extrusion molding methods for producing extrusion molded articles suitable as stock shapes for machining, the method using resin materials that contain general-purpose resins and/or engineering plastics, have been proposed so far.
For example, Japanese Unexamined Patent Application Publication No. 2005-226031A (Patent Document 1) discloses a method for producing a stock shape for machining having a thickness or diameter exceeding 3 mm, the method comprising solidification- and extrusion-molding a resin composition containing an engineering plastic such as a polyether ether ketone, polyetherimide, polyphenylene sulfide, polysulfone, polyether sulfone, or polycarbonate.
On the other hand, biodegradable plastics have drawn attention as polymer materials that have little adverse effect on the environment, and have been used in applications including extrusion molded articles such as films and sheets, blow molded articles such as bottles, injection molded articles, and the like. Recently, application of biodegradable plastics in stock shapes for machining has been increasingly demanded.
Polyglycolic acid is a crystalline resin having superior tensile strength, tensile elongation, bending strength, elastic modulus in bending, hardness, flexibility, heat resistance, and the like compared to other biodegradable plastics such as polylactic acid, and the polyglycolic acid is also a biodegradable plastic having greater or equal gas barrier properties to general-purpose gas barrier resins. Polyglycolic acid can be molded into films and/or sheets via extrusion molding. For example, Japanese Patent No. 4073052B (Patent Document 2) discloses a method for molding polyglycolic acid into a sheet via extrusion molding. In the disclosure, various sheet molded articles are produced using the sheet, having a thickness of 0.01 to 5 mm, utilizing its toughness, heat resistance, transparency, and other characteristics.
Furthermore, Japanese Unexamined Patent Application Publication No. 2010-069718B (Patent Document 3) discloses a solidification- and extrusion-molded article of polyglycolic acid, having a thickness or diameter of 5 to 100 mm, that is produced by subjecting polyglycolic acid to solidification- and extrusion-molding. Specifically, a solidification- and extrusion-molded article of polyglycolic acid having a density of 1.575 to 1.625 g/cm3 and a thickness or diameter of 5 mm or greater but 100 mm or less, the solidification- and extrusion-molded article of polyglycolic acid being formed of a resin material containing polyglycolic acid having a melt viscosity of 10 to 1,500 Pa·s, particularly preferably 70 to 900 Pa·s, measured at a temperature of 270° C. under a shearing speed of 120 sec−1, has been disclosed. Patent Document 3 describes that, if the thickness or diameter is too large (i.e. the thickness or diameter exceeds 100 mm), it will be difficult to sufficiently remove or reduce residual stress even when the solidification- and extrusion-molded article is heat-treated, and machining a solidification- and extrusion-molded article having a large residual stress tends to cause deformation in the obtained secondarily molded product.
If an extrusion molded article that is thicker and suitable as a stock shape for machining such as cutting can be obtained by using polyglycolic acid, which is a degradable plastic, it will be possible to provide a secondarily molded article having excellent properties, leading to development of new applications of polyglycolic acid.
To retrieve hydrocarbon resources (in the present invention, also simply referred to as “petroleum”) from ground containing hydrocarbon resources such as petroleum (e.g. shale oil) and gas (e.g. shale gas), a downhole (underground bore hole) is provided. Use of degradable plastic in downhole tools or components thereof (i.e. downhole tool components; e.g. mandrel of a plug for petroleum excavation or the like is well known), which are components used to form or maintain the downhole or to promote the retrieval of the resources, is expected since degradable plastic can be disintegrated in the downhole without collecting it on the ground after use.
For example, relatively small ball sealers that have a diameter of 16 to 32 mm and that are formed from non-degradable materials such as aluminum and/or non-degradable resins such as nylon and phenol resins which are coated, as necessary, with rubber to improve sealing properties has been conventionally used as ball sealers (which is an example of a downhole tool) used to fill bore holes. However, in recent years, demands for ball sealers or the like having a larger diameter (e.g. diameter of 25 to 100 mm or even greater) have been increasing as a part of components constituting a downhole tool (downhole tool component), such as plugs, including frac plugs and the like, and frac sleeves (tube and plug for hydraulic fracture), used in hydraulic fracturing. When a ball sealer or the like having such a large diameter is produced by injection molding or compression molding using a degradable resin, which is often a crystalline resin, sink marks and/or voids are caused due to thermal shrinkage after the production or shrinkage associated with crystallization, and dimensional precision required for ball sealers, which is a filling component, or the like is not obtained. In order to obtain a ball sealer or the like having a relatively large diameter, production of a ball sealer or the like by cutting a solidification- and extrusion-molded article having a large thickness or diameter formed of degradable resin, for example, has been attempted. However, as described above, when the thickness or diameter of the solidification- and extrusion-molded article of polyglycolic acid, which is a degradable resin, exceeds 100 mm, it is difficult to sufficiently remove or reduce residual stress, the obtained secondarily molded product tends to be deformed, and, in some cases, breaks or cracks readily occur. Therefore, a solidification- and extrusion-molded article of degradable resin having excellent strength, processability, and the like as well as having a sufficiently large thickness or diameter to obtain a ball sealer or the like having a relatively large diameter has been demanded.